1. Field of the Invention
The present invention relates to a permanent-magnet motor used in compressors of domestic refrigerators and air conditioners or other various kinds of industrial machinery, and, more particularly, to a permanent-magnet motor that is capable of approximating a back electro motive force wave to a sinusoidal wave, thereby lowering noise and increasing efficiency.
2. Description of the Related Art
Generally, a permanent-magnet motor, such as a brushless DC motor, has permanent magnets mounted at a rotor core to generate a rotary driving force. Based on how the permanent magnets are mounted at the rotor core, permanent-magnet motors are classified into surface-mounted permanent-magnet motors and embedded permanent-magnet motors.
The embedded permanent-magnet motor has a plurality of permanent magnets mounted in the rotor core. In the embedded permanent-magnet motor, dispersion of the permanent magnets is structurally prevented using magnet torque as well as reluctance torque. Consequently, the embedded permanent-magnet motor provides a higher efficiency than the surface-mounted permanent-magnet motor, which has permanent magnets mounted on the surface of the rotor core to create a magnetic field, and therefore, the embedded permanent-magnet motor is used when high-speed rotation is required.
Based on how coils are wound on a stator core, embedded permanent-magnet motors are classified into coil concentration-type motors and coil distribution-type motors. FIG. 1 is a cross-sectional view illustrating a conventional permanent-magnet motor constructed in accordance with a coil distribution-type winding method, and FIG. 2 is a cross-sectional view illustrating another conventional permanent-magnet motor constructed in accordance with a coil concentration-type winding method.
Referring to FIGS. 1 and 2, the conventional permanent-magnet motor comprises: a stator 1a,1b on which coils are wound; and a rotor 4a,4b rotatably disposed in the stator 1a,1b. 
The stator 1a,1b comprises: a stator core 2a,2b formed by stacking a plurality of magnetic steel sheets in the shape of a cylinder; a plurality of slots 3a,3b formed at the stator core 2a,2b while being arranged in a circumferential direction; and a plurality of coils wound on the slots 3a,3b. 
The rotor 4a,4b comprises: a rotor core 5a,5b formed by stacking a plurality of magnetic steel sheets in the shape of a cylinder, the rotor core 5a,5b being disposed in a hollow part of the stator 1a,1b while being spaced a predetermined distance from the hollow part of the stator 1a,1b; a plurality of permanent magnet insertion holes 6a,6b formed at the rotor core 5a,5b while being arranged in the circumferential direction; and a plurality of permanent magnets 7a,7binserted in the permanent magnet insertion holes 6a,6b, respectively. A rotary shaft 8a,8b is inserted in a hollow part formed at the center of the rotor 4a,4b, and is thereby is rotated along with the rotor 4a,4b. 
When electric current is supplied to coils wound on the slots 3a,3b of the stator 1a,1b of the conventional permanent-magnet motor with the construction described above, polarities of the coils are sequentially changed. Therefore, a rotary magnetic field is generated at teeth 9a,9b of the stator 1a,1b formed between the slots 3a,3b. Consequently, a magnetic field is created at the rotor 4a,4b, in which the permanent magnets 7a,7b are embedded while being adjacent to the teeth 9a,9b. The magnetic field of the rotor 4a,4b follows the rotary magnetic field generated at the teeth 9a,9b of the stator 1a,1b, and, therefore, the rotor 4a,4b is rotated along with the rotary shaft 8a,8b to generate a rotary driving force.
In the embedded permanent-magnet motor, a back electro motive force wave, induced at the coils wound on the stator slots 3a,3b, contains many high harmonic components depending upon the location and shape of the permanent magnets 7a,7bembedded in the rotor 4a,4b. However, the length of a gap 94a,94b between the surface of each of the teeth 9a,9b and the outer circumferential surface of the rotor 4a,4b is uniform. As a result, a change in magnetic flux at the gap 94a,94b according to the rotation of the rotor 4a,4b is made in the shape of a non-sinusoidal wave. Therefore, the back electro motive force wave is distorted, as shown in FIG. 3. Consequently, a non-sinusoidal back electro motive force wave is created, and, therefore, torque ripple is increased. As a result, vibration is generated when the rotor 4a,4b is rotated, and noise is increased due to the vibration. Consequently, efficiency of the motor is lowered.